The present invention generally relates to rotary head type digital signal reproducing apparatuses, and more particularly to a rotary head type digital signal reproducing apparatus for playing a magnetic tape which is pre-recorded with a tracking reference signal in predetermined starting and ending portions of each track which is formed obliquely to the longitudinal direction of the magnetic tape and with a digital audio signal in an intermediate portion between the starting and ending portions of each track.
In a digital audio tape recorder, an analog audio signal is modulated into PCM audio data by a pulse code modulation (PCM), and the PCM audio data are recorded on and reproduced from a magnetic tape. In a rotary head type digital audio tape recorder which employs rotary magnetic heads, data are successively recorded on and reproduced from tracks formed obliquely to a longitudinal direction of the magnetic tape without a guard band between two mutually adjacent tracks, alternately by a pair of rotary heads having gaps of mutually different azimuth angles. A tracking reference signal is recorded on and reproduced from starting and ending portions of each track, while the PCM audio data are recorded and reproduced from an intermediate portion between the starting and ending portions of each track with a predetermined signal format.
The PCM audio data are recorded and reproduced with at least two kinds of modes. In a linear standard mode, the PCM audio data have a sampling frequency of 48 kHz, two channels and a quantization number of sixteen bits. On the other hand, in a non-linear long-time mode (hereinafter referred to as a half-speed mode), the PCM audio data have a sampling frequency of 32 kHz, two channels and a quantization number of twelve bits. Actually, there are other non-linear modes such as a mode in which the PCM audio data have a sampling frequency of 44.1 kHz, four channels and a quantization number of twelve bits, however, these other modes all have the same play time as the standard mode.
In the half-speed mode, the rotational speed of a rotary drum on which the rotary heads are mounted and the tape transport speed are respectively set to speeds which are one-half those in the standard mode. In addition, the frequencies of digital signals (more accurately, the frequencies of clock pulses for producing the PCM audio data and a the tracking reference signal) are set to one-half those in the standard mode. In other words, the operation speed of the digital audio tape recorder as a whole in the half-speed mode is set to one-half that in the standard mode, except for a part of the digital audio tape recorder where a conversion is carried out between the analog audio signal and the digital signal.
The data rate in the standard mode is 48 (kHz).times.2.times.16=1536 (kbits/sec), and the data rate in the half-speed mode is 32 (kHz).times.2.times.12=768 (kbits/sec). Accordingly, the sound quality obtained in the half-speed mode is slightly deteriorated when compared to that obtained in the standard mode, but there is an advantage in that the play time in the half-speed mode is two times that in the standard mode for a given length of the magnetic tape because the operation speed of the digital audio tape recorder in the half-speed mode is set to one-half that in the standard mode.
It is desirable that the half-speed mode is added to the rotary head type digital audio tape recorder having the standard mode. However, due to the following problems, the realization of such a digital audio tape recorder is difficult costwise and technically, and would cause deterioration in the quality of the digital audio tape recorder.
Firstly, the tracking reference signal frequency and the carrier frequency of the PCM audio data in the half-speed mode become one-half those in the standard mode. For this reason, particularly in the reproducing mode, the operation frequencies of an analog filter circuit part and a phase locked loop (PLL) circuit part for reading data within a signal processing circuit for processing the tracking reference signal and the PCM audio data must be switched between the standard and half-speed modes. Alternatively, it is necessary to provide a circuit part exclusively for use in the standard mode and another circuit part exclusively for use in the half-speed mode.
Secondly, the coupling between the rotary heads mounted on the rotary drum and a recording amplifier and a reproducing amplifier is normally made through a rotary transformer. However, since the signal frequency in the half-speed mode becomes one-half that in the standard mode, the coupling in the low frequency range becomes deteriorated in the half-speed mode.
Thirdly, when the setting is made so that an optimum carrier-to-noise ratio is obtained in the standard mode, an output voltage of the reproducing rotary head in the half-speed mode becomes one-half that in the standard mode because the relative linear velocity between the magnetic tape and the rotary head in the half-speed mode is one-half that in the standard mode. As a result, according to this setting, the carrier-to-noise ratio becomes deteriorated in the half-speed mode.
Fourthly, a drum motor for rotating the rotary drum and a capstan motor for rotating a capstan which drives the magnetic tape must have predetermined performances in the two rotational speeds corresponding to the standard and half-speed modes, where the predetermined performances refer to the tolerable range of the jitter in the rotation of the rotary drum for maintaining phase synchronism between the rotation of the rotary drum and an electrical circuit, the tolerable range of instability of the rotation of the capstan motor and the like.